Turn Your 27″ iMac Into An Awesome Monitor

I didn’t intend to buy a new iMac. I was happy with my Mac Mini desktop system and my MacBook Pro was a fine portable (now that the video has been repaired). I simply wanted more display real estate so I could do some video editing. But my shopping trip took a detour at the Apple store: The 27″ iMac is simply the best computer monitor you can buy, and it happens to have a great computer embedded in it too! The gorgeous LED IPS panel and massive 2560 x 1440 resolution really shines in intensive applications.

There is another secret lurking inside, though: The Mini DisplayPort connector on the back is bi-directional, so you can connect another computer, turning that new 27″ iMac into a gorgeous monitor! I was eager to try this out, and sure enough my 2009 Mac Mini had no trouble taking over the iMac’s display using a $30 Belkin cable. But actually using the iMac in this configuration has not been pleasant.

The Basics

First, only “late 2009″ 27” iMacs have this display input capability. Although the ports on the back of the 21.5″ iMac and others look identical, none can do this trick.

Get your hands on a 27″ iMac, a Mini DisplayPort cable (I used this $30 Belkin cable), and a recent Mac Mini, MacBook, or other computer with a Mini DisplayPort video output. Plug one end into the iMac and the other into your other Mac. As soon as the two Macs are running, the display will blank out and switch from the iMac to the external one.

You can switch inputs by pressing Command-BrightnessUp (usually Command-F2 or Command-Fn-F2) on the iMac’s keyboard. It will also automatically switch when it detects a signal, such as when a computer is connected or wakes up from sleep.

While the external source is displayed, the iMac will ignore most keyboard and mouse movements. Some special function key commands appear to continue working, however, including brightness (F1/F2), rewind (F7), play/pause (F8), fast forward (F9), and mute/volume (F10/F11/F12).

The Limitations

Sound good? Well, there are some serious limitations that make this not so great in practice.

1. I can’t get any USB keyboard to trigger the display switch no matter what I try. Only the original Apple BlueTooth keyboard that came with the iMac will trigger a switch. This annoys me, since I’m a dedicated IBM Model M user…
2. There is no KVM capability in OS X, so the keyboard and mouse are useless when you connect an external computer. I use an IOGear USB KVM and was able to get this working with my Mac Mini and iMac, but switching required the otherwise unused BlueTooth keyboard (see above).
3. The iMac has to be on and running OS X for the Mini DisplayPort input to function.
4. Switching sources takes a second or two, and the iMac switches displays whenever the connected computer wakes from sleep. This leads to some surprising work pauses if you leave it connected.
5. Only native DisplayPort video is usable. Although there are many cheap Mini DisplayPort adapters, these are normally for output only and will not work. So computers with VGA, DVI, or HDMI ports won’t work. without a converter.

I tried all sorts of tricks to send the proper key sequence from my USB keyboards and using AppleScript, but nothing worked. I’d love for Apple to deliver a KVM solution in OS X, but barring that an enterprising programmer ought to be able to figure out a software solution.

Can It Be Used With Other Computers?

The short and easy answer is no, the iMac’s Mini DisplayPort input is very unlikely to work with anything but a Mac, since most non-Macs don’t use DisplayPort. And since DisplayPort is a computer (rather than entertainment) standard, it’s unlikely that your Blu-Ray player, game machine, or DVR will work either.

The medium answer is yes, it might very well work just fine with some PCs equipped with a DisplayPort video card.

The longer answer is yes, a variety of companies are working on adapters to allow other devices to use the iMac’s display. Apogee has promised a special converter box just for this exact purpose, and folks have had varying success with the Gefen and Atlona converters.

The very long answer is that you can force just about anything to work with the iMac’s Mini DisplayPort input but it’ll be very hard and probably not worth it. There are three things to consider when trying to send video from one device to another:

1. The physical/electrical connection must be compatible. The iMac only accepts Mini DisplayPort cables, so any connected device must terminate with a male Mini DisplayPort connector. Most basic (read cheap) DVI or HDMI adapters can not convert formats, and merely connect appropriate pins from one format to another. There are plenty of cable and adapter options for regular DisplayPort, however. But DisplayPort 1.1 fiber optic cables are right out.
2. The data format (including DRM) must be compatible. Simply using a DVI-to-Mini DisplayPort adapter is insufficient. The DVI device will send DVI signals that the iMac won’t be able to interpret, or they might not be connected at all. Even if the data format is converted correctly, it appears that the iMac does not support HDCP input, although HDCP is enforced for signals output to HDTVs. This means that a DVI Blu-Ray player might not display correctly on the iMac even if the DVI signal is correctly converted!
3. Display resolution is less of a concern, but must be considered as well. My iMac was happy with a variety of input resolutions from my Mac Mini, but the native 2650×1440 resolution is way beyond the DVI spec. So full resolution will require a dual-link DVI connection (expensive and rare) or a scaler (even more expensive).

What does this mean? Here’s the answer for various devices:

1. Apple computers with Mini DisplayPort connectors just need a cable, including the 2009 MacBook, MacBook Pro, Mac Mini, and iMac.
2. Other computers might work, provided they have MiniDisplayPort or plain large DisplayPort, though the latter requires a physical adapter.
3. Computers with DVI connectors (including older Macs) will probably need an expensive adapter like the Gefen, Atlona, or Apogee boxes and won’t be able to use the full resolution of the iMac without an expensive scaler box like this one from Atlona. We’re talking $100 to $500 to get things connected, and it might not work.
4. Entertainment devices like Blu-Ray players, Xboxes, and PlayStations likely require HDCP enforcement so they probably won’t work without some special anti-DRM voodoo. This is exactly what Apogee is talking about delivering, but I’ll believe it when I see it. And it’ll likely cost at least $200. Note that this is spelled explicitly out in the DisplayPort FAQ!

Conclusion

Finally there is the issue of sound and control. The iMac’s Mini DisplayPort input only supports video, so audio and USB connectivity requires an alternative path. You can use a KVM like my IOGear to switch your keyboard and mouse, but no audio will be sent. Probably the best compromise would be to use external speakers for both the iMac and connected device.

So there you have it. The iMac’s awesome display can be used by external devices. But it’s not really satisfying to use without some extra devices: A KVM and speakers at least, and some expensive video voodoo at most. Sorry to ruin your day!